Heretofore, as a die used for extrusion of a ceramic honeycomb, a honeycomb-molding die is known which has such a structure that groove-like slits are formed on the front surface in cell blocks and body introduction holes communicated with the slits are formed on the rear surface.
In order to adjust the widths of the slits of the cell blocks and improve the durability of the die, such a honeycomb-molding die is produced by, for example, a method of coating a stainless base material with a titanium-based film having excellent abrasion resistance (film containing, as components, one or more materials selected from the group consisting of TiC, TiN and TiCN) by CVD or PVD.
At present, when a honeycomb-molding die having a base material coated with a titanium-based film is worn, regeneration of the honeycomb-molding die is carried out primarily by immersing the honeycomb-molding die in a solution so as to remove the remaining titanium-based film, coating the base material with the titanium-based film again, and then making a pattern adjustment.
In this case, as the above solution, there are generally used (1) a removing solution containing 60 to 70% of nitric acid as a main component (refer to Japanese Patent Application Laid-Open No. 109126/1997), (2) a removing solution containing 35% or lower of hydrogen peroxide as a main component, and (3) a removing solution containing hydrogen fluoride as a main component.
However, all of these removing solutions are intended to be used to remove a relatively small amount of titanium-based film. Thus, they have the following problems in removing a large amount of titanium-based film when a wide surface area is coated with a relatively thick titanium-based film as in the case of the honeycomb-molding die.
Firstly, although the removing solution containing nitric acid as the main component has great power to dissolve titanium, titanium ions in nitric acid are liable to change into an oxide and be deposited. Thus, when the amount of titanium-based film to be dissolved is large as compared with the amount of nitric acid, titanium ions dissolved in nitric acid once are deposited as an oxide.
As a result, when the removing solution containing nitric acid as the main component is used, an oxide of titanium is re-deposited on the surface of a base material, thereby making it impossible in some cases to recoat the base material after removal.
Further, since the oxide of titanium is stable, it cannot be removed by the removing solution containing nitric acid as the main component, once it is deposited on the surface of the base material.
Secondly, although the removing solution containing hydrogen peroxide as the main component has great power to dissolve and retain titanium, it is decomposed into water and oxygen due to the presence of metal ions dissolved from a base material or the like. Thus, titanium ions dissolved once are deposited as the oxide of titanium due to decomposition of hydrogen peroxide.
For this reason, when a titanium-based film is removed by use of hydrogen peroxide, a large amount of the removing solution must be used in consideration of the decomposition of hydrogen peroxide.
Thirdly, since the removing solution containing hydrogen fluoride as the main component is highly caustic, it is inappropriate to use the removing solution for removing a titanium-based film formed on a stainless base material since it corrodes even the base material.
The present invention has been conceived in view of such problems of the prior art. An object of the present invention is to provide a method capable of removing a large amount of titanium-based film as well as the oxide of titanium adhered/deposited on the base material of a honeycomb-molding die by use of a small amount of the removing solution without corroding the base material of a honeycomb-molding die while preventing re-deposition of dissolved titanium ions on the base material.